With the aim of saving energy and protecting the environment, it is intended to bring into general use the installation of automatic start/stop systems in automobiles.
The principle of operation of these systems consists, in certain conditions, of bringing about the complete stop of the heat engine when the vehicle is itself at a standstill, then of restarting the heat engine as a result, for example, of an action of the driver which is interpreted as a request to restart.
In contrast to traditional starters formed by electrical motors which are more or less perfected, the electrical machines used in these “Stop and Go” systems are generally reversible, i.e. able to function either as starter or as alternator. The replacement of two distinct machines by a single machine assists the optimisation of the use of the energy available on board the vehicle.
In a familiar manner, the alternator/starter is either linked to the flywheel of the heat engine, being integrated therewith, or brought into rotation by the crankshaft by means of a transmission by pulleys and belt, for example.
The characteristics of the alternator/starter, generally of polyphase machines, are optimised according to the mode of operation required in controlling in an appropriate manner the currents which feed the phases.
One example of the method of controlling a rotating polyphase reversible electrical machine for an automobile with a heat engine is described in French Patent Application FR2854746.
The method of control described in this document allows one to obtain an optimum torque depending on the rotational speed of the machine when it functions as an electric motor (starter mode or auxiliary motor mode), including high speed.
However, in this method, the excitation current is constant and equal to a nominal current applied during starting, namely when it is found that the more the speed increases, the less suitable a maximum excitation current is for maximising the torque or the output.
The method described is thus not totally adapted to new uses of alternators-starters in automobiles, such as dynamic assistance (the electrical machine obtains temporary additional power during overtaking, for example), or the accompaniment of the heat engine in a stop phase in order to limit vibrations, which requires precise suitability, for all engine speeds, of the torque supplied by the electrical machine to the torque of the heat engine to which it is mechanically coupled.